Closed-loop pre-coding has been introduced in the Long Term Evolution (LTE) Release 8 (Rel-8) to improve the spectrum efficiency. Closed-loop pre-coding firstly requires the same group of pre-coding matrixes, referred to as a codebook, to be stored at both a network side device and a user equipment. The user equipment estimates channel information from a common pilot signal of a cell and then selects a pre-coding matrix from the codebook according to some criterion which can be the maximizing the mutual information, maximizing the output signal to interference and noise ratio, etc. The user equipment sends an index of the selected pre-coding matrix in the codebook to the network side device over an uplink channel, where the index is referred to as a Pre-coding Matrix Indicator (PMI). The network side device can determine from the value of the received index of the pre-coding matrix to be used for the user equipment. The pre-coding matrix reported by the user equipment can be considered as a quantized value of channel state information.
In an existing cellular system, an array of antennas of a network side device is typically arranged horizontally as illustrated in FIG. 1 and FIG. 2. A beam at a transmitter of the network side device can be adjusted only horizontally but the beam is transmitted with a common vertical down tilt value for each user equipment, so various beam-forming/pre-coding technologies are generally applied based upon horizontal channel information. In fact, since a radio signal propagates in three dimensions in space, the performance of the system may not be optimal with common vertical down tilt value. Vertical adjusting of the beam may be of great significance to the improved performance of the system. Along with the development of antenna technologies, an array of active antennas with each array element being separately controllable has emerged in the industry as illustrated in FIG. 3A and FIG. 3B. Vertical dynamic adjusting of a beam becomes possible with this array of antennas. 3D beam-forming/pre-coding may be performed in a Frequency Division Duplex (FDD) system based upon channel state information reported by a user equipment, possibly using a codebook as conventionally used in the LTE Rel-8 system.
However this report of the channel state information may suffer from the following problems:
1. An array of active antennas with each array element being separately controllable is applied for antenna elements in 3D beam-forming, and antenna power amplifiers are integrated together with the antenna elements, so there is very low transmit power of each antenna element when there are a large number of antenna elements. If a pilot signal is transmitted over each antenna element as done in the prior art, then there may be so low transmit power that the user equipment might fail to achieve correct signal measurement, and the performance of data transmission.
2. If a pilot signal is transmitted over each antenna element, then there will be a such a large number of antenna ports, for each of which the user equipment has to make signal measurement and calculate Channel State Information (CSI) from a signal measurement value, that the complexity of the user equipment will be high when there are a large number of antenna elements.
In summary, as in the prior art, then there will be low transmit power of each antenna unit, and the complexity of the user equipment will be high when there are a large number of antenna elements.